The communication capacity by wireless communication through a mobile terminal such as a mobile phone, a smart phone, and a mobile router has been increasing year after year as the versatility of terminal apparatus has increased, and the distribution contents such as movie files or music files have been improved. The wireless communication technique has been developed to meet the above demands. Currently, the base station facilities or various terminal apparatuses compatible with the fourth generation (4G) communication standards have become widespread, and widely used in public.
In the aforementioned wireless communication terminal apparatuses, a signal transmitted or received through an antenna is called a radio frequency (RF) signal which has a high frequency. When an RF signal is received, the RF signal is multiplied by an output signal from a local oscillator to down-convert, and is converted to a band called a baseband which includes information to be actually communicated in an RF front-end circuit. When transmitting information, a baseband signal is multiplied by an output signal from the local oscillator to up-convert, and is sent as an RF signal via an antenna.
Generally, an oscillator such as a voltage controlled oscillator (VCO) is used as the local oscillator. The VCO is an oscillation circuit that controls output frequencies by a control voltage to be input. In general, the VCO is used as the local oscillator in which the control voltage is generated by a phase locked loop circuit, and a frequency error caused by various factors in a frequency of an output signal of the VCO is corrected.
For example, Patent Literature 1 describes a phase locked loop circuit in that an output of the VCO is input to an analog-to-digital converter (ADC), phase comparison is performed based on the converted digital data, and a control voltage of the VCO based on the comparison is output so as to stabilize the frequency.
In the communications of the fourth generation and onward, multi-value modulation such as 256 quadrature amplitude modulation (QAM) is used. For the use of the multi-value modulation, it is necessary to maintain the frequency of the local oscillator to be constant. Accordingly, the frequency of the local oscillator is stabilized by the phase locked loop circuit.
In addition, in the aforementioned fourth generation communication standards, the efficiency of use of the frequency band is increased by performing communication using multiple subcarriers, which is called an orthogonal frequency-division multiple access (OFDMA). The use of the local oscillator that has high spectral purity can prevent interference between subcarriers, and can increase the communication capacity by utilizing the frequency band more efficiently.
An example of an oscillator that has higher spectral purity is a surface acoustic wave (SAW) oscillator. The SAW oscillator is used with a phase locked loop circuit by stabilizing the output frequency. For frequency stabilization of the SAW oscillator, a method of adjusting an application voltage to a varicap (variable capacitance diode) is mainly used.
In addition, a multiple-input and multiple-output (MIMO) has been known as a technique for increasing the amount of wireless communication. This technique is to increase the communication capacity by using a transmitter and a receiver for wireless communication both of which have multiple transmission/reception systems each including an antenna, a modulator and a demodulator.